In portable electronic devices, information is often stored in volatile memory, for example random access memory or RAM. This type of memory can operate, that is information can be written or stored into RAM, and read out of RAM, at rates substantially similar to the rate at which a processor is operating. A drawback of RAM is that in general it requires electrical power to not only operate, but also to maintain information. In other words, if power is removed from the RAM device, it will loose all information stored within.
As an unfortunate consequence, in a battery operated device, when the energy from the device's battery has been consumed, the information stored in RAM may be lost. In most cases the information created and collected by the user and stored in the device is much more valuable than the device itself. Obviously, losing such information would limit the commercial viability of such products.
There have been numerous prior art attempts to overcome this problem. One prior art method uses a back-up battery that provides energy only for the limited function of maintaining the contents of RAM while the main battery is being replaced or recharged.
Another prior art method uses a “lock out” mechanism to reserve a portion of energy in a main battery for the sole purpose of maintaining the contents of RAM. In this mode, after a threshold level has been reached, e.g., 90% of the battery's energy has been expended, the user can no longer initiate operation of the device. The remaining 10% of the energy is used solely to maintain the contents of RAM. After the final 10% is consumed, RAM contents are lost.
Yet another prior art method combines the aforementioned “lock out” with a super capacitor. During the lock out mode, the dead battery may be removed and replaced with a fresh cell. The super capacitor is capable of supplying enough energy to maintain the contents of RAM for perhaps one minute, which should be sufficient time to replace the battery.
Unfortunately, the methods discussed above, all suffer from the same fundamental drawback, that is, maintaining the contents of RAM still consumes power, and there is only a limited amount of power available in the device. If the user does not replenish the energy (replace or recharge the battery) before the “lock out” reserve or the back up battery or the super cap runs out, the information is still lost.